Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional dual magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional dual MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional first antiferromagnetic (AFM) layer 14, a first conventional pinned layer 16, first a conventional tunneling barrier layer 18, a conventional free layer 20, a second conventional tunneling barrier layer 22, a second conventional pinned layer 24, conventional second AFM layer 26, and a conventional capping layer 28. Also shown is top contact 30.
Conventional contacts 11 and 30 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. The conventional tunneling barrier layers 18 and 22 are nonmagnetic and are, for example, thin insulators such as MgO.
The conventional pinned layers 16 and 24 and the conventional free layer 20 are magnetic. The magnetizations 17 and 25 of the conventional pinned layers 16 and 24, respectively, are fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the corresponding AFM layers 14 and 26. Although depicted as a simple (single) layer, the conventional pinned layers 16 and 24 may include multiple layers. For example, the conventional pinned layers 16 and/or 24 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used.
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. Although shown as in-plane, the magnetization 21 of the conventional free layer 20 may have a perpendicular anisotropy.
To switch the magnetization 21 of the conventional free layer 20, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven between the top contact 30 and the bottom contact 11, the magnetization 21 of the conventional free layer 20 may switch to be parallel or antiparallel to the magnetization 17 of the conventional pinned layer 16. When a sufficient current is driven from the bottom contact 11 to the top contact 30, the magnetization 21 of the free layer may switch to be antiparallel to that of the pinned layer 16. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
Although the conventional dual MTJ 10 may be written using spin transfer and used in an STT-RAM, there are drawbacks. Use of the conventional dual MTJ 10 allows for a lower write current. However, for a high tunneling magnetoresistance (TMR) both a good separation of resistance area (RA) values between the tunneling barriers 18 and 22 as well as a low total RA for the conventional MTJ 10 are desired. For example, an RA of less than 5 Ω-μm2 is desired. In addition, a factor of five to ten between the RAs of the tunneling barriers 18 and 22 is desired. This translates to a thickness of on the order of 1 nm for the tunneling barrier layers 18 and 22. Because of these requirements, it may be difficult to manufacture a high quality tunneling barrier. For example, achieving a tunneling barrier 18 or 22 that is continuous, has the desired crystal structure, and the desired orientation may be problematic. Use of the conventional magnetic dual MTJ 10 may also have other drawbacks. For example, a read current driven through the conventional dual MTJ 10 may disturb the state of the free layer 20. For example, if the magnetization 21 of the conventional free layer 20 is in the −x direction in FIG. 1, then a read current driven through the conventional dual MTJ 10 in the z direction may result in a spin torque tending to switch the free layer magnetization 21 in the +x direction. Some portion of the conventional dual MTJs 10 in a magnetic memory may then be switched. In addition, the conventional dual MTJ 10 may be subject to stagnation for spin transfer-based switching. In particular, as a current is driven through the conventional dual MTJ 10, there is initially no spin torque because the charge carriers are aligned parallel or antiparallel to the magnetization 21 of the free layer 20. Thus, there is a stagnation point in switching which is undesirable. Once the magnetization 21 starts precessing, there is a torque on the magnetization 21 and the free layer 20 may be switched.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories, preferably without sacrificing the lower switching current achieved through the use of a dual MTJ. The method and system described herein address such a need.